1. Field of the Invention
The present invention relates to a gas sensor control apparatus for controlling a gas sensor which detects the concentration of a gas.
2. Description of the Related Art
Conventionally, a gas sensor has been used which is mounted on a vehicle and is attached to an exhaust pipe of an internal combustion engine, and which detects the concentration of a specific gas contained in an exhaust gas, for the purpose of, for example, controlling an air-fuel ratio. Known examples of such a gas sensor include an oxygen sensor for detecting the concentration of oxygen and an NOx sensor for detecting the concentration of nitrogen oxide (NOx). Such a gas sensor often uses a solid electrolyte member mainly made of zirconia (zirconium oxide) in a sensor element section. Such a solid electrolyte member becomes active when heated to a high temperature (about 600° C. or higher), and exhibits excellent oxygen ion conductivity. In view of this, in a gas sensor in which a solid electrolyte member is used in the sensor element section, a heater section for heating the sensor element section is provided so as to heat the sensor element section to an activation temperature at which the sensor element section becomes active. In order to maintain the sensor element section at a proper activation temperature, the element resistance having a certain relation with the element temperature of the sensor element section is detected, and the supply of electric current to the heater section is feedback-controlled such that the element resistance coincides with a target resistance.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No 2006-113081
3. Problems to be Solved by the Invention:
Incidentally, in order to perform the above-mentioned heater energization under feedback control in such a gas sensor, it is necessary to detect the element resistance of the sensor element section regularly, while controlling the energization of the heater section. One known method of detecting the element resistance of the sensor element section is temporarily changing the voltage between the electrodes of the sensor element section or the current flowing between the electrodes, detecting the change in voltage or current in response to the temporal change (the magnitude of the change will be referred to as a response change amount), and detecting the element resistance from the response change amount. Meanwhile, since on-off control (PWM control) is performed through use of pulses in order to control the energization of the heater section, switching noise may be generated at the leading edge and trailing edge of each pulse; namely, at timings at which the state of heater energization is switched from the OFF state to the ON state and switched from the ON state to the OFF state. Accordingly, if the timing of switching the heater energization state overlaps with a period during which the response change amount is detected by the above-described method, the switching noise affects the detected response change amount, and in some cases the element resistance cannot be detected accurately.
In order to solve such a problem, for example, Patent Document 1 discloses a heater control apparatus for a gas concentration sensor which is configured such that when the element resistance detection period and the timing of on/off switching of heater energization overlap each other, the detection period or the switching timing is forcedly shifted.
However, such a control apparatus must always monitor the detection period and the switching timing and determine whether or not they overlap each other. When the detection period and the switching timing are expected to overlap each other, the control apparatus must adjust the detection period or the switching timing so as to prevent the occurrence of such overlap. Therefore, control tends to become complex, and there is a need for simpler, more reliable measures against such overlap between the detection period and the switching timing. In particular, there is a need for measures against irregular overlap at a timing which changes with a change in pulse width caused by feedback control, the timing being one of timings corresponding to the leading edge and trailing edge of each pulse.